Argentite

The name refers to the high-temperature form of silver sulfide, only stable over 177° C. The mineral acanthite is the stable form of this mineral below 177° C. This is the tarnish that forms on sterling silver. An important ore of silver, it is found in moderately low-temperature hydrothermal sulfide veins and in zones of secondary enrichment. Its name comes from the Latin "argentum" meaning “silver.”

Mineral Photos courtesy of R.Weller/Cochise College